Upper Air Measurements

Transcription

1 Upper Air Measurements

2 Upper Air Measurements Measurements above the surface become increasingly difficult with al;tude Balloons, airplanes and rockets have all been used to carry instruments alo? to make measurements Ground- and Satellite- based instruments can be used to make measurements What por;on of the atmosphere does upper air refer to in meteorology?

3 Methods for Measurement Measurements of temperature, pressure, humidity and wind can be made using in- situ instrument packages as well as remotely Remote sensors can be classified as ac;ve sensors or passive sensors

4 Ac;ve Remote Sensors Sensors emihng radia;on/radio frequencies to measure a remote property are considered ac;ve sensors Radar, Lidar

5 Passive Remote Sensors Emit no radia;on/radio frequencies. Receive only emioed signals Microwave radiometers, some satellite imagery

6 In- situ versus Remote Why use in- situ when remote is readily available? How good is remote coverage?

7 In- situ PlaRorms Aircra? Balloons Radiosondes Theodolites

8 Aircra? Measurements Take measurements along their flight path Commercial aircra? as well as research aircra? can be ourioed with equipment Research aircra? generally equipped with high- end instrumenta;on and can be manned or unmanned Commercial aircra? use simple packages to measure temperature, humidity and wind What is an obvious problem with commercial measurements?

9 Balloons Categorized according to size, color and extensibility Most balloons are the extensible type Inextensible balloons are also known as constant level balloons Color only maoers if the balloon is to be observed visually (light preferred on clear days and dark on cloudy days)

10 Balloon Types Pilot balloons used to track horizontal winds Carry no payloads Usually weigh between 10 and 100 grams Sounding balloons Carry a payload to measure winds, temperature, humidity and pressure Weigh grams Larger balloons available for bigger payloads

11 Balloon Gases What gases are commonly used to fill the balloon? What are the advantages/disadvantages to each?

12 Balloon Flight How high do balloons get? What happens to radiosonde package?

13 Balloon Burst Height

14 Drag Force Why isn t drag force shown on the prior plot?

15 Balloon Ascent Rate Func;on of li?, weight and payload Total li? force on a balloon Total li? mass

16 Balloon Ascent Rate Free Li? Force Free Li? Mass

17 Balloon Ascent Rate Balloon Drag Reynolds Number What is Dynamic Viscosity?

18 Dynamic Viscosity

19 Drag Coefficient file:///.file/ id=

20 Steady State Ascent When is steady state ascent achieved?

21 Balloon Ascent Rate If the balloons al;tude is known, the density and dynamic viscosity can be calculated The Standard Atmosphere chart can also be used

22 Standard Atmosphere

23 Balloon Ascent Rate Balloon ver;cal speed

24 Calcula;on Errors Balloon materials are not completely elas;c, thus balloon volume is not inversely propor;onal to atmospheric density as assumed Gas inside the balloon will leak through the balloon As balloon ascends into cooler air, thermal lag may cause internal gas to be warmer than external gas

25 Calcula;on Errors Balloon shape is not always spherical Ver;cal wind components can aid or deter buoyant effects Drag coefficient is not constant and changes with density, diameter and ascent speed of balloon

26 Average Ascent Rate Typical ascent rate is m/s and is usually nearly constant with al;tude

27 Wind Measurement Horizontal wind speed can be determined by visually tracking a balloon (pilot balloon) Tracking capabili;es include theodolites, double theodolites and radars Use of an instrument package can include tracking using GPS Balloons follow the mean flow because of their large surface area and small mass

28 Wind Force Force exerted on a balloon by the wind force If we assume the balloon speed is approximately equal to wind speed, we can simplify

29 Theodolites Measures the azimuth of the balloon (rela;ve to true north) and the eleva;on angle usually at 1- minute intervals If the ascent rate is known, the posi;on of the balloon can be determined and wind velocity inferred from successive posi;ons Two theodolites increase tracking accuracy Limited to visual range Works poorly with cloud cover

30 Balloon Tracking Calcula;ons

31 Radiosonde Tracking Transmits measured data to ground via radio transmioer Posi;on can be determined using a radio theodolite or radio direc;on finder Combine pressure from radiosonde with tracking informa;on and complete posi;on can be computed Radio theodolites work best with high- frequency radiowave sondes (typically Mhz)

32 Radar Tracking Balloons carrying corner reflectors can be tracked by radar Determines slant range to the balloon as well as azimuth and eleva;on angles Measures angles to within 0.1 degrees and to within 30 meters. Balloon height can be corrected for curvature of the earth

33 Naviga;on Aids Some radiosondes carry special radio receivers that detect naviga;onal signals Data is relayed to a ground sta;on where posi;on is then calculated Requires more expensive sonde, but less expensive ground sta;on (neither radar nor radio direc;on finder is needed) Ideal for portable ground sta;ons Naviga;onal aids available are LORAN C, OMEGA, and GPS

34 LORAN C LORAN LOng Range Aid to Naviga;on High Accuracy, medium- range naviga;onal aid opera;ng at frequencies around 200 khz Primary use is marine naviga;on

35 OMEGA Network of eight atomic- clock- controlled transmioers opera;ng in the very- low- frequency band Designed to provide global coverage Each sta;on transmits sequen;ally for 0.9 to 1.2 seconds on three assigned frequencies (10.2, 11.3, 13.6 khz) No two sta;ons transmit at the same ;me on one frequency

36 OMEGA No individual sta;on transmits on more than one frequency at a ;me Cycle is repeated every 10 seconds At the three frequencies, the earth s surface and the ionosphere act as waveguides TransmiOers excite various modes of propaga;on whose amplitudes and phase veloci;es vary with the height of the ionosphere, direc;on of propaga;on and range from transmioer

37 GPS Array of 24 satellites orbi;ng at km above the earth s surface Orbital period 12 hours and the orbital plane is inclined at 55 degrees with respect to the equatorial plane Each satellite is equipped with an atomic clock and transmits signals at precise intervals on two frequencies (1.226 GHz and GHz)

38 GPS If a receiver can hear four satellites at once, the posi;on of the receiver can be determined to within 100 m (worst case) Works anywhere on the earth s surface Sonde transmits the received GPS signals to its ground sta;on which also receives GPS signals Ground sta;on computes the posi;on of the sonde and of itself and obtains the sondes posi;on rela;ve to the ground sta;on

39 GPS This procedure (called differen;al GPS) compensates for many of the system errors

40 Naviga;onal Aid Errors LORAN C Wind speed errors: 0.7 m/s Ver;cal posi;on: 150 m OMEGA Wind speed errors: 1.5 m/s Ver;cal posi;on: 300 m GPS Wind speed errors: 0.1 m/s Ver;cal posi;on: 30 m

41 Radiosondes Carry sounding payloads to heights of 30 km or higher Typically measure pressure, temperature and humidity and sends data to a ground sta;on via radio transmioer Special payloads have been designed to measure radia;on (solar and terrestrial), ozone concentra;on, electrical poten;al gradient, air conduc;vity, and radioac;vity

42 Radiosonde Requirements Generally designed to be used only once Must be designed for mass produc;on at low cost Must be capable of opera;ng on an internal baoery for ~ 3 hours Must be able to transmit radio signal at least 200 km Mass density must be low enough not to damage jet engines or windshields

43 Radiosonde Requirements Have a short service life, but need to have a long shelf life before use Dri? must be carefully controlled or sensors must be calibrated quickly before flight To obtain readings representa;ve of layers ~ 100 meters thick, a reading from each sensor must be completed every seconds for a 5 m/s ascent rate

44 Pressure Sensor Usually a single metallic diaphragm aneroid or silicon diaphragm sensor Designed to respond to the logarithm or pressure rather than linearly with pressure to enhance high- al;tude performance Metallic diaphragm sensors lose sensi;vity at low pressure (< hpa) so they are some;mes augmented with hypsometers (why?)

45 Temperature Sensors Small- rod thermistors or temperature- sensi;ve capacitors Thermal lag must be small (because of constant changing pressures and densi;es) Typical ;me constant for a rod thermistor 1.27 mm in diameter is 4.5 seconds at 1000 hpa, 10.6 seconds at 100 hpa, and 30 seconds at 10 hpa

46 Temperature Sensors Radia;on error is a major concern Sensor is ven;lated by 5 m/s ascent of sonde, but must not be exposed to direct sun Radia;on shield is part of sensor itself and consists of highly reflec;ve coa;ng Doesn t use typical shield like the sensors on the ground

47 Humidity Sensors sensors are the typical humidity sensors used in radiosondes Psychrometric sensors have been used on some and chilled- mirror hygrometers have been used for research projects Response ;me increases as temperature decreases because of the lower amounts of water vapor present at colder temperatures

48 Dropsondes Created by scien;sts and engineers at NCAR Designed to be dropped from an aircra? at al;tude to more accurately measure (and therefore track) tropical storm condi;ons as the device falls to the ground The dropsonde contains a GPS receiver, along with pressure, temperature, and humidity (PTH) sensors to capture atmospheric profiles and thermodynamic data

49 Dropsondes The device's descent is usually slowed by a parachute, allowing for more readings to be taken before it reaches the water beneath Dropsondes are commonly used by Hurricane Hunters to obtain data on hurricanes, and these data are then fed into supercomputers for numerical weather predic;on

50 Dropsondes The dropsonde is released when the plane reaches the eye of the hurricane, normally at around 10,000 feet (approx. 3,000 meters)

51 Dri?sonde A dri?sonde is a high al;tude, durable weather balloon holding a transmioer and a bank (35 in the first models) of miniature dropsonde capsules These water- boole- sized transmioers have enough power to send informa;on on their parachuted fall to the balloon, and then the balloon holds a transmioer powerful enough to relay readings to a satellite

52 Dri?sondes The sensor packages are rela;vely inexpensive ($400 each) A?er being introduced in April 2007, around a thousand a year are expected to be used to track winds in hurricane breeding grounds off of West Africa, which are too far for prac;cal opera;on of Hurricane Hunter planes

53 Dri?sondes

54 Tethersondes Uses a balloon tethered to the ground Regular intervals along the length of the tether are instrumented to provide measurements of temperature, humidity, pressure and winds

55 Tethersondes

56 Radiosonde Exposure Errors Accentuated by extreme varia;ons in temperature, pressure, air density, solar radia;on, electric field gradients, environmental shock in handling, wehng and icing by cloud drops, etc Generally best to make sensors as small as possible to counteract problems Sonde package swings below balloon and the pendulum mo;on can be detected by some systems which need to be filtered out

57 Barometer Errors Small size means smaller temperature gradients across the sensor and faster response to temperature changes Silicon integrated circuit a few mm in thickness and diameter can be used

58 Temperature Sensor Errors Small size needed to reduce radia;on hea;ng and dynamic lag Vaisala RS- 90 sonde uses an unshielded capaci;ve sensor 0.1 mm in diameter Radia;on hea;ng is approximately 0.5 K at 10 hpa which can be corrected down to 0.1 K

59 Humidity Sensor Errors Capaci;ve sorp;on sensors in some sondes are designed to eliminate condensa;on in supercooled clouds Other sondes use dual sensors to measure RH; one is heated to remove condensate while the other is measured Hea;ng is then switched to the other and measurements are taken on the first one a?er cooling Hea;ng cycle is ~ 40 seconds

60 Temperature and RH Sensor Errors Both are located on an arm protruding from the sonde to isolate them from heat from the sonde and provide beoer atmospheric exposure